The subject invention is directed toward the art of pipe or tube joints and couplings and, more particularly, to a flange-type tube joint incorporating an improved relationship between the face seal and an associated gasket.
Flange-type tube joints have soft material gaskets, e.g. elastomeric, plastic, etc. that are clamped between opposed flange end faces for sealing purposes. These tube joints are frequently used in hygienic processing systems where fluid retention is highly undesirable. These systems are usually constructed with small angle slopes on the order of 2.degree. on tubing runs and employ gravity drains. Therefore, due to the small angle slope, it is necessary to limit the potential for fluid retention by concentrating on regions where fluid retention may occur. For example, it is known to design the gasket and associated flanges such that when the joint is completed, the radially inner face of the gasket forms a substantially flush or smooth continuation between the interior surfaces of the connected tubes. Joints of this general type are shown, for example, in U.S. Pat. Nos. 3,775,832, 4,735,445, and German Patentschrifts CH 674764 A5 and 947,034.
The prior art has proposed various methods or designs to assure that over or under compression of the gasket cannot occur. In spite of this, it has been found that both situations can occur resulting in a variety of process problems that are significant and often unrecognized. When gaskets are over compressed the gasket material is extruded into the flow path of the pipe or tubing system. This can happen during initial makeup of the fitting or following a period of elevated process temperatures, such as a steam sterilization cycle, when thermal expansion of the gasket will result in excessive extrusion. Excessive extrusion causes several primary problems including flow restriction, cell shear, system contamination and fluid retention. Flow restriction can occur, for example, from having the inner periphery of the gasket extend excessively into the flow path thus creating an orifice effect. Hygienic systems are validated by the FDA for specific cleaning procedures including minimum flow rates needed to create the turbulent flow conditions required to effect the cleaning process. A gasket in a restrictive flow condition can cause the system to not meet the required fluid velocities and consequently not meet government regulations. A second problem, cell shear, is caused by delicate micro-organisms impacting on the extruded gasket resulting in the destruction of the cell which is the target product of the bioprocess. A third problem, system contamination, can result when over time of exposure to process conditions, especially steam, an extruded gasket can become brittle and break off pieces that cause system contamination. A fourth problem, fluid retention, occurs when draining the system to recover the product. The extruded gaskets result in retention of substantial amounts of process fluid in the system. This is a significant problem in systems requiring a high degree of cleanliness and/or ease of flushing and changeover between process fluids.
In the case of under compression two primary problems occur, fluid entrapment and leakage. Under compression can occur from improper assembly or as a result of thermal cycles. As a gasket extrudes under temperature, as discussed above, when cooled back to normal or ambient process temperatures, it will not return to its original position in conventional hygienic fittings. When this condition occurs there is less compressive force on the remaining gasket material within the fitting which can cause the fitting to loosen. In some cases only a single thermal cycle can result in a loss of the seal at the bore line which allows process fluid to enter the seal area and become entrapped. The isolated fluid cannot be cleaned without disassembly and can cause contamination of the same or subsequent batches. In some cases, process fluid reaches atmosphere and leakage occurs.